MB-OFDM (Multiband-Orthogona Frequency Division Multiplexing) is particularly used in a wireless communication system such as UWB (Ultra-WideBand) system. MB-OFDM is usually used in combination with a frequency hopping technology.
MB-OFDM is suitable for a device which is designed to satisfy the Federal Communications Commission (FCC) regulations on UWB devices. According to the current regulations, a UWB device is allowed to operate in the frequency bandwidth from 3.1 to 10.6 GHz, with the emission of power always kept under −41.3 dBm/MHz within the signal bandwidth.
In a particular MB-OFDM system, coded bits are transmitted in the form of an OFDM symbol, each containing QPSK (Quadrature Phase Shift Keying) or DCM (Dual Carrier Modulation, or tone), and a pilot tone.
As a result, the resultant waveform is a frequency hopping waveform in which each OFDM symbol corresponds to the hops in the transmission frequency bandwidth. A local oscillator generally maintains the same frequency setting during the entire transmission period.
FIG. 1 shows a frequency hopping pattern of a MB-OFDM system. Referring to FIG. 1, each rectangle represents an OFDM symbol which carries 200 bits (or 100 bits). Three hop bandwidths (f1, f2, f3) facilitate frequency hopping among the symbols.
Each of the symbols 100, 102, 104 is modulated using carriers which are respectively different from each other according to the frequency bandwidths (f1, f2, f3). Accordingly, patterns repeats with respect to the next three symbols (106, 108, 110).
The frequency hopping technology is advantageous in reducing interference between neighboring UWB devices. However, this technology accompanies the following several problems. When the dat is transmitted with lower rate than supported in the system, generally, the data bits are copied and modulated for transmission.
Accordingly, the same data bits can be modulated several times in the same subcarrier frequency, and as a result, the copied symbols fall into the same multipath and fading conditions. Accordingly, frequency diversity decreases.
Additionally, because the neighboring data bits usually have the same frequency, or are usually coded using the adjacent subcarrier, this also deteriorates frequency diversity. Accordingly, a data bit coding method, which can improve frequency diversity of not only the same data, but also the adjacent data bits, is demanded. Also, a coding method, which does not accompany a considerable amount of additional costs and increase of complication with existing transmitter and receiver, is required.